1. Field of the Invention
This invention relates to a laser-diode-pumped solid state laser in which a solid laser crystal is pumped by a laser diode (semiconductor laser), a semiconductor laser and a radiation image read-out system, and more particularly to a laser which causes harmonics from a solid laser crystal to oscillate in laser oscillation and a radiation image read-out system.
2. Description of the Related Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as "a stimulable phosphor". It has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a stimulable phosphor sheet (a recording medium provided with a layer of the stimulable phosphor). The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is used for reproducing the radiation image of the object as a visible image on a recording medium such a photosensitive material or a display such as a CRT. See U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication Nos. 56(1981)-11395 and 56(1981)-11397 and the like.
The radiation image recording and reproducing system is practically advantageous in that as compared with conventional radiographies, an image can be recorded over an extremely wide radiation exposure range.
When reading out radiation image information from the stimulable phosphor sheet in the radiation image recording and reproducing system, for instance, a light beam such as a laser beam is caused to two-dimensionally scan the stimulable phosphor sheet storing thereon a radiation image, and the light emitted from the stimulable phosphor sheet upon stimulation thereof is transmitted to a photodetector such as photomultiplier through an optical guide having a light inlet end face extending along the main scanning line. The photodetector detects in time series the light emitted from the stimulable phosphor sheet upon stimulation thereof and an image signal made up of image signal components for respective picture elements is obtained.
A red laser diode whose output power is about 30 mW at most has been employed as a stimulating ray source in the radiation image recording and reproducing system described above. In order to read out image information stored on a stimulable phosphor sheet and to obtain a reproduced image of high quality, the stored energy of radiation must be converted into light at a high efficiency and for this purpose, a higher output power stimulating light is required.
As disclosed, for instance, in Japanese Unexamined Patent Publication No. 62(1987)-189783, there has been known a laser-diode-pumped solid state laser in which a solid state laser crystal doped with a rare earth element such as neodymium is pumped by a light beam produced by a laser diode. Further in such a laser, a laser beam shorter than the fundamental wave in wavelength or a higher output laser beam can be obtained by disposing a nonlinear optical crystal in the resonator to selectively cause harmonics such as a second harmonic (SHG) of the solid laser beam to make laser oscillation. The harmonics can be as high as not lower than 300 mW in output power.
However it has been found that stability of such a second harmonic or the like is apt to be affected by the resonator and especially the laser output is instable until the resonator is thermally stabilized.
That is, as shown in FIG. 3A, when a current not less than the threshold above which laser oscillation is generated (e.g., 3 to 4 A) is directly poured into a pumping laser diode in an off state (in a state where no current is poured), the laser power of the second harmonic (SHG) is increased sharp but the laser power is once stabilized at a power about half of full power (300 mW in the example shown in FIG. 3A) before reaching the full power and then increased sharp to the full power. The time t1 required for the laser output to be stabilized at its full power after the pumping laser is turned on (will be referred to as "rise-up time", hereinbelow) is, for instance, 3 to 4 seconds in the SHG lasers though depending upon the laser medium and the current poured into the pumping laser diode.
An approach for suppressing time loss due to the rise-up time in the case where rapid processing is required as in the radiation image recording and reproducing system described above is to keep the SHG laser continuously operating. With this approach, since the SHG laser is constantly kept on, the time loss due to the rise-up time can be nullified. However, on the other hand, such continuous operation of the SHG laser results in an extreme reduction in the life of the laser and is undesirable from the viewpoint of economy.